Box springs for beds are generally made of a rigid, rectangular base frame and a wire grid frame supported in spaced, parallel relation by a plurality of interposed spring units spaced transversely and longitudinally of the frames. Two types of springs are used, for example, coil springs and bent wire springs. Spring assemblies where all of the springs are coil springs are prone to be too soft and, hence, insufficiently supported at the sides and ends, while spring assemblies comprised of the bent wire type are prone to be too stiff and to damage the mattress. It is the purpose of this invention to provide a spring assembly wherein both coil springs and bent wire springs are used with the bent wire springs distributed along the sides and ends where pressure is concentrated, as contrasted to the central area where the pressure is distributed, but not so stiff as to damage the mattress. A further object of the invention is to provide a bent wire spring unit of improved construction structured to be snapped onto the grid frame as are the coil springs shown in U.S. Pat. No. 4,004,304. Another object is to provide bent wire springs structured to be used in spring assemblies exclusively of coil spring units.